People frequently use unmanned, self-propelled vehicles such as robots to perform a variety of functions that would be difficult or dangerous for a person to perform. For example many people frequently use robots to retrieve or dispose an explosive device or inspect or work in an environment that could kill or injure a person. People also frequently use robots to inspect or work in locations that typically are hard to access or are inaccessible by a person such as inspecting a pipeline.
Unfortunately, because robots typically propel themselves to a work site, use of most conventional unmanned, self-propelled vehicles is typically significantly limited by the ability of the robot to propel itself over a surface. For example, surfaces that include compound curves or three dimensional curves, abrupt inclinations or declinations, steps or gaps can cause conventional robots to become significantly less stable, i.e., more likely to lose their preferred orientation relative to the surface, as they traverse the surface or turn on it. In addition, surfaces that are slippery can cause conventional robots to easily lose a significant portion, if not all, of their traction to the surface. If either happens while traversing an incline or inverted surface such as a ceiling, such a loss of traction could cause the robot to fall. Such a fall could seriously damage the robot, its payload if it has any, or the surface or other components of the structure the robot is traversing.
Another problem with conventional robots is they tend to scrub the surface as they traverse and turn on it. This can cause undesirable scratches on the surface. For example, the skin or windshield of a commercial airplane must remain free from scratches because of the high stress imposed on it during flight. If a scratch does occur, the skin or windshield is often replaced at great expense in both time and money.
Yet another problem with conventional robots is they tend to bounce or jerk as they propel themselves across a surface. This can be a significant problem during inspection of, for example, a commercial airplane's crown skin or structure—the top part of the airplane's body—because most inexpensive non-destructive inspection techniques require the inspection apparatus to remain a substantially constant distance from the surface being inspected. Because of this requirement, most inspections of an airplane typically include erecting a scaffold, which can be time consuming, for an inspector to stand on prior to inspecting the structure.